This experiment was conducted to investigate the effects of Macleaya cordata extract (MCE) on growth performance and immunity of weaner piglets. A total of 60 piglets weaned at 35 days of age reproduced by PIC hybrid sows with similar body weight, parity, milk yields and litter size were designed randomly to 5 groups, and each had 4 replicates with 3 piglets per replicate. Control group was not added the MCE, experimental groups were fed basal diets supplemented with 2.5, 5.0 and 7.5 mg/kg MCE, respectively, and terramycin group was fed the basal diet with adding 50 mg/kg of terramycin. The duration of the experiment was 25 days. Growth performance, serum glutamic-oxalacetic transaminease (GOT) and glutamic-pyruvic transanease (GPT) activities, contents of urea nitrogen (UN), albumin and immunoglobulin G (IgG), lysozyme (LSZ) activity and nitric oxide (NO) content of piglets were determined, respectively. The results indicated that at 35 to 60 days of age, average daily gain and feed to gain ratio of weaner piglets showed a quadratic curve change with the increase of MCE supplementation (P<0.05); average daily gain and average daily feed intake of piglets in 5.0 mg/kg MCE group were significantly higher than those in control group and terramycin group (P<0.05), and feed to gain ratio was significantly lower than that in the control group, 7.5 mg/kg MCE group and terramycin group (P<0.05). At 60 days of age, serum GOT activity in 5.0 and 7.5 mg/kg MCE groups was extremely significantly lower than that in control group (P<0.01), and serum GOT activity in terramycin group was extremely significantly lower than that in control group, 5.0 and 7.5 mg/kg MCE groups (P<0.01), but serum GPT activity was extremely significantly higher than that in control group (P<0.01). Serum UN content in 5.0 mg/kg MCE group was extremely significantly lower than that in control group, 2.5 and 7.5 mg/kg MCE groups and terramycin group (P<0.01). At 60 days of age, serum IgG content in 5.0 mg/kg MCE group was extremely significantly higher than that in control group, 2.5 and 7.5 mg/kg MCE groups and terramycin group (P<0.01). Serum IgG content in terramycin group was extremely significantly higher than that in control group (P<0.01). Serum NO content in 2.5 and 5.0 mg/kg MCE groups was extremely significantly higher than that in control group and terramycin group (P<0.01), and for control group and 7.5 mg/kg MCE group, there were no significant differences (P>0.05). Serum NO content in terramycin group was extremely significantly higher than that in control group (P<0.01). Serum LSZ activity in 5.0 and 7.5 mg/kg MCE groups was extremely significantly higher than that in control group and 2.5 mg/kg MCE group (P<0.01). Serum LSZ activity in terramycin group was extremely significantly higher than that in control group and 2.5, 5.0 and 7.5 mg/kg MCE groups (P<0.01). It is suggested that the MCE can enhance the contents of serum IgG, NO and LSZ activity, and improve immunity, disease-resistant ability and growth performance of weaner piglets. MCE effects are remarkably superior to terramycin. Based on the results of quadratic curve of growth performance and serum antibacterial indexes, adequate supplementation of MCE in weaner piglet diets is 4.5 mg/kg.